Brain irradiation is a powerful tool in the treatment of cancers including primary and metastatic many tumors, and even leukemias and lymphomas. However its use is restricted by the poor tolerance of the nervous tissue and the development of serious side effects among long-term survivors. These include a decline in IQ and cognitive function. This phenomenon is very frequent and irreversible. Its pathogenesis is not fully elucidated but it involves extensive demyelination occurring in a delayed fashion (months to years). Based on preliminary data obtained in the rat, we hypothesize that radiation exhausts the pool of dividing oligodendrocyte progenitors (OPC) and therefore the brain's ability to replace the myelinating cells. We have developed significant experience in the neural differentiation of human ES (hES) cells and we propose the transplantation of hES-derived neural precursors as an effective strategy for repair of radiation-induced brain damage. [unreadable] [unreadable] We will study the temporal and topographic pattern of cell loss following whole brain irradiation in the rat at five timepoints over the course of one year. The rats will receive BrdU prior to sacrifice and the brains will be processed for BrdU, NG2 (an OPC marker) and other neural markers. We will then graft irradiated rats with two different populations of neural precursors obtained from hES cells: a multipotential neural stem cell-like population, and an oligodendroglial progenitor population. In our preliminary data we have successfully derived such cells from hES cells and established stable eGFP-expressing hES cells via lentiviral gene transfer for reliable in vitro and in vivo identification. Grafted animals will be examined at 6 weeks after transplantation to assess cell survival, migration, and ability to replace the OPC niche, and at 6-8 months after transplantation to evaluate the ability of hES cells to contribute to remyelination of the injured brain. This work addresses a therapeutically important problem in brain cancer treatment and will provide a wealth of information on the behavior of grafted human ES cells within the context of an injured brain. If successful, we plan to initiate future studies addressing the functional impact of hES-derived neural precursor grafts on radiation-induced cognitive decline. This grant will only use the INIH-approved cell line: HI. NIH code WA01. [unreadable] [unreadable]